Azo dyes bearing water solubilizing groups have long been known as dyestuffs for natural polyamide fibers. Typically such azo dyes have contained at least one sulfonic acid group, although a relatively minor amount of the research in azo dyes for natural polyamide fibers has been devoted to sulfonic acid-free azo dyes which contain at least one carboxylic acid group. It is generally recognized in the art that such azo dyes are generally unsuited for the dyeing of synthetic fibers such as polyethylene terephthalate.
We have now discovered a novel dyeing process which makes use of the thermofixation process and a specific class of azo dyes which yields excellent dyeings on polyesters, particularly polyethylene terephthalate, with azo dyes containing a free carboxylic acid group.